The effrontery of the band? (5)
I believe the answer is:
brass
I believe this is a double definition.
'the effrontery' is the first definition.
('brass' can be a synonym of 'effrontery')
'the band?' is the second definition.
(brass band is a kind of band)
'of' is the link.
(Other definitions for brass that I've seen before include "neck" , "Family of wind instruments" , "Alloy; money" , "An alloy; part of orchestra" , "Common alloy" .)